1. Field of the Invention
The present invention generally relates to nanoparticle synthesis and assembly, and more particularly to chemical reduction of metal salts to form alloy nanoparticle materials.
2. Description of the Related Art
The CoPt and FePt binary alloy materials have long been known for their permanent magnetic applications. Recently, CoPt or FePt based nanoparticle materials have been predicted to be the next generation of ultrahigh density recording media because they are both chemically stable and magnetically hard. [D. Weller, A. Moser, IEEE Trans. Magn., 35, 4423 (1999)]. The megnetocrystalline anisotropy (Ku), a term measuring the hardness to flip the magnetization of a single magnetic domain, can reach to the order of 108 erg/cm3. Compared to 106 erg/cm3 for current cobalt-based recording media, this high Ku indicates that the particles can be reduced to about 2.8 nm but still show ferromagnetic properties needed for magnetic recording. These hard magnetic nanomaterials also find applications in magnetic bias films of magneto resistive elements, and magnetic tips for magnetic force microscopy. [S. H. Liu, IEEE Trans. Magn., 35, 3989 (1999); S. H. Liu, Y. D. Yao, J. Magn. Mag. Mater, 190, 130 (1998)].
Various vacuum-related techniques have been developed in making high coercivity M/Pt alloy particle thin films [see, for example, U.S. Pat. No. 6,007,623; U.S. Pat. No. 5,989,728; U.S. Pat. No. 5,824,409; and U.S. Pat. No. 5,846,648], the complete disclosures of which are herein incorporated by reference. These include conventional sputtering [K. R. Coffey, M. A. Parker, J. K. Howard, IEEE Trans. Magn. 31, 2737 (1995); C. P. Luo and D. J. Sellmyer, IEEE Trans. Magn. 31, 2764 (1995); M. Watanabe and M. Homma, Jpn. J. Appl. Phys. 35, L1264 (1996); T. Suzuki, N. Honda, K. Ouchi, J. Appl. Phys. 85, 4301 (1999)]; co-sputering [M. R. Visokay and R. Sinclair, Appl. Phys. Lett. 66, 1692 (1995); N. Li and B. M. Lairson, IEEE Trans, Mag., 35, 1077 (1999)]; molecular beam epitaxy [B. M. Lairson, M. R. Visokay, R. Sinclair, B. M. Clemens, Appl. Phys. Lett. 62, 639 (1993); A. Cebollada et al., Phys. Rev. B 50, 3419 (1994); T. C. Hufnagel, M C. Kautzky, B. J. Daniels, B. M. Clemens, J. Appl. Phys. 85, 2609 (1999)]; and ultrahigh vacuum deposition [S. Mitani et al., J. Magn. Mag. Mater. 148, 163 (1995); G. Dumpich et al., J. Magn. Mag. Mater. 161, 37 (1996)], the complete disclosures of which are herein incorporated by reference.
The main problem among all of these common procedures, however, is the wide distribution of as-deposited grain size and uncontrolled agglomeration of the post-annealed magnetic grains. This uncontrolled aggregation will limit the further recording density increase, as variously sized magnetic grains will result in a dramatic noise increase of the recording signal.
Recently, solution phase chemical synthesis was developed to make M/Pt nanoparticles. It is believed that the solution can offer the important homogenous nucleation step and facilitate isotropic growth of the nuclei suspended in the solution, and will yield monodisperse magnetic nanoparticle materials. These chemically made monodisperse magnetic nanoparticles can be used as building blocks to fabricate functional devices such as ultrahigh density magnetic recording [U.S. Pat. No. 6,162,532; S. Sun, D. Weller, J. Mag. Soc. Jpn, 25, 1434 (2001)], the complete disclosures of which are herein incorporated by reference.
One method of chemical synthesis includes metal salt reduction in reverse micelles at room temperature [E. E. Carpenter, C. T. Seip, C. J. O""Connor, J. Appl. Phys., 85, 5164 (1999)], the complete disclosure of which is herein incorporated by reference. Another method of chemical synthesis includes redox transmetalation reaction in warm toluene solution [J. -I. Park, J. Cheon, J. Am. Chem. Soc., 123, 5743 (2001).]. One disadvantage of these methods is that due to the low reaction temperatures, these methods generally yield particles that are poorly crystallized and irregular in their internal structure. A third solution phase synthesis of M/Pt nanoparticles involves a combination of high temperature decomposition of metal carbonyl precursor, such as Fe(CO)5, and reduction of platinum salt in the presence of surfactants [U.S. Pat. No. 6,254,662; S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science, 287, 1989 (2000)], the complete disclosures of which are herein incorporated by reference.
However, metal carbonyl may not be an ideal starting material in certain circumstances due to its toxicity. Thus, there is a need for the production of nanoparticle materials from non-toxic metal salts. Therefore, it is necessary to have an alternate procedure that can reduce metal salts to create alloy nanoparticles with controlled particle sizes.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods of producing high coercivity alloy particle thin films, the present invention has been devised, and it is an object of the present invention to provide a method for synthesizing M/Pt alloy nanoparticles, with controlled particle sizes and coercivity, by chemically reducing metal salts. In order to attain the object suggested above, there is provided, according to one aspect of the invention a process for making M/Pt nanoparticles via metal salt reduction.
Specifically, the method for making M/Pt nanoparticles via metal salt reduction comprises, first, mixing metal salts in a solvent. Second, a reducing agent is added to the solvent at a temperature in the range of 100xc2x0 C. to 350xc2x0 C. Third, the M/Pt nanoparticle dispersion is stabilized. Fourth, the M/Pt nanoparticles are precipitated from the M/Pt nanoparticle dispersion. Finally, fifth, the M/Pt nanoparticles are re-dispersed into the solvent. The metal salt comprises a combination of M salt and Pt salt with M salt derived from any one of FeCl2, Fe(OOCCH3)2, Fe(CH3COCHCOCH3)3, CoCl2, Co(OOCCH3)2, or Co(CH3COCHCOCH3)2, and Pt salt derived from Pt(RCOCHCOR)2 (R comprises an alkyl group), and PtCl2. The reducing agent comprises one of MBR3H, MH, metal naphthalides, polyalcohol, such as R(OH)2; wherein R comprises an alkyl group, and wherein M comprises one of Li, Na, and K. The step of stabilizing the nanoparticle dispersion further comprises utilizing a long chain carboxylic acid and amine; wherein the carboxylic acid is RCOOH and the amine is RNH2, wherein R is a C8 chain and greater (longer). Moreover, the solvent comprises one of aromatic ether, dialkyl ether, and trialkyl amine, wherein the aromatic ether comprises diphenylether, and the dialkyl ether comprises one of dibutyl ether and dioctylether. Additionally, the trialkyl amine comprises one of tributyl amine and trioctyl amine. The method comprises adding a co-surfactant to facilitate nanoparticle growth, wherein the co-surfactant comprises alkyl alcohols. Moreover, addition of a co-reducing agent to facilitate nanoparticle separation also occurs, wherein the co-reducing agent comprises alkyl alcohol, wherein said alkyl alcohol comprises one of RCHCH2(OH)2 and CnH2nxe2x88x921OH, wherein R comprises an alkyl group, and wherein n is in the range of 4 to 22.
Alternatively, the reducing agent may be added to the solvent at a temperature in a range of 50xc2x0 C. to 350xc2x0 C. Furthermore, the metal salt (M1/M2) may comprise a combination of FeCl2, FeCl3, Fe(OOCR)2, Fe(RCOCHCOR)3, CoCl2, Co(OOCR)2, Co(RCOCHCOR)2, NiCl2, Ni(OOCR)2, Ni(RCOCHCOR)2, Pt(RCOCHCOR)2, PtCl2, Pd(OOCR)2, Pd(RCOCHCOR)2, and PdCl2, wherein R comprises an alkyl group.
Also, the reducing agent may comprise one of MBR3H, MH, M naphthalides, Rxe2x80x94HNxe2x80x94NH2, RCHO and R(OH); wherein R comprises one of H and an alkyl group, and wherein M comprises one of Li, Na, and K. Moreover, the step of stabilizing the nanoparticle dispersion may comprise utilizing a long chain carboxylic acid and amine; wherein the carboxylic acid is RCOOH and the amine is RNH2, wherein R is a C6 chain and greater. Furthermore, the solvent may comprise one of aromatic ether, dialkyl ether, and trialkyl amine, wherein the aromatic ether may comprise phenylether, wherein the dialkyl ether may comprise dioctylether, and wherein the trialkyl amine comprises tributyl amine and trioctyl amine.
Also, a co-surfactant may be added to facilitate nanoparticle growth, wherein the co-surfactant may comprise one of alcohol and ROH, wherein R comprises an alkyl group. Moreover, the co-reducing agent may comprise alkyl alcohol, aldehyde and amine, wherein the alkyl alcohol comprises ROH, alkyl aldehyde comprises RCHO, and alkyl amine comprises RHNNH2, and wherein R comprises a C-based long chain group.